The present invention relates to a nuclear reactor cooled by a liquid metal of the type comprising a vessel filled with liquid metal, whose upper part is sealed by a rigid slab which supports the heat exchangers and the pumps of the primary circuit.
More specifically the invention relates to an integrated fast neutron nuclear reactor, i.e. of the type in which the complete primary circuit of the reactor is housed in the vessel. In this type of reactor, the core thereof containing the fuel assemblies rests on the bottom of the vessel or on its periphery by means of a support for the supply of liquid metal (generally sodium) and a plate system. The liquid sodium is heated in the core by the fission reaction of the nuclear fuel before entering a hot collector placed above the core. It then circulates in heat exchangers in which it transmits a large part of its heat to the fluid (generally sodium) flowing in the secondary circuits. The cooled primary sodium leaving the lower part of the heat exchangers in a cold collector is sucked in by primary pumps, which reinject it into the support. In this type of reactor, the exchangers and primary pumps are generally suspended on the slab sealing the reactor vessel. The same applies with regards to a certain number of other members necessary for the operation or safety of the reactor, including the exchangers for cooling the reactor when it is shut down.
In known manner the slab sealing the vessel is constituted by a welded sheet metal structure forming a group of concrete-filled cavities in order to constitute a neutron protection and contribute to the rigidity of the slab. The lower face of the slab is provided with a cooling circuit and a lower thermal insulation covering immersed in the neutral gas above the liquid metal at approximately 500.degree. C. This face can then be kept at a temperature which does not exceed e.g. 100.degree. C. The thickness of the slab, which is fixed in such a way that an adequate strength and displacements which are sufficiently reduced during the temperature variations of the lower face are obtained is approximately 2.50 m for large reactors with an electric power of 1000 to 1500 MW. The exchangers and primary pumps are installed in a group of orifices or vertical shafts passing through the slab and having a diameter which is sufficient to permit their vertical introduction. In their part corresponding to the thickness of the slab, these components have sealing, as well as thermal and neutron protection members ensuring a functional continuity with the slab. The heads of these components, which are specific to their operation (motors for the pumps, connections to the secondary circuits, etc.) are positioned above the slab, as well as the system of circuits. A slab formed in this way is considered to be thin because every effort is made to reduce the heights devoted to neutron insulation and supporting functions, both with respect to the slab in order to facilitate the overall design and on the components, whose total height considerably influences the cost and ease of handling.
A structure formed in this way with a thin slab in stages and an upper area for the heads of the components suffers from several disadvantages. Firstly the group of equipment and heads of components above the slab are vulnerable to impacts and shocks taking place during the handling of heavy objects above the slab. In addition, the thus heightwise exposed heads of exchangers can only be protected from possible secondary sodium leaks leading to a fire by adding a doubling envelope, which is onerous and makes it more difficult to regularly inspect the main wall. Moreover, for a given strength, the thinness of the slab makes it necessary to use more steel than if freedom existed with regards to the thickness. Furthermore, to limit heightwise displacements linked with expansion of the lower plate, this thinness imposes severe constraints with regards to its temperature. Finally the cavities within the slab, which are entirely filled with concrete cannot be inspected.